The demand for organic acids, such as lactic acid, citric acid, ascorbic acid, gluconic acid or fumaric acid, has been increasing over the years, owing to their extensive use in food, pharmaceutical, detergent or biodegradable plastic industries. The fermentation processes achieve the production of organic acids at the industrial scale. Depending on the pH requirement of the bacteria strain used, the organic acids produced from the fermentation process is largely in salt form. The recovery of the organic acids from fermentation broth is a challenge to separation specialists.
Traditional process for recovery and purification of organic acids from fermentation broth generally involves one or several precipitation stage. For example, under the common industrial process for lactic acid production, the fermentation broth is generally heated to ca. 70° C. to kill the bacteria and then acidified with sulfuric acid to pH 1.8. The precipitated salt, which mainly constitutes of gypsum and biomass are removed by filtration and the resulting liquid is treated with activated charcoal to remove any coloring materials. The clarified liquid is then ion exchanged and concentrated to 80%. Smell and taste can be further improved by oxidative treatment, e.g., with hydrogen peroxide. The lactic acid obtained at this stage is usually of consumable quality but not suitable as pharmaceutical grade. Several additional purification steps are necessary to achieve that. The greatest disadvantage of the traditional process is the high loss of lactic acid during the crystallization steps.
Alternative downstream processing techniques have been researched for more environmental friendly downstream processing.
Several electrodialysis membrane technologies have been proposed as the key steps for recovery and purification of lactic acid. One possible way is to de-mineralized the lactic acid directly by using conventional electrodialysis membrane (i.e. cation & anion exchange membrane), where lactate salt (the broth, e.g. sodium lactate) is running in one stream, acid solution (e.g. hydrochloric acid) running in another, with two water streams running in between of the broth and the acid stream within the electrodialysis stack. The lactate passing through the anion exchange membrane combines with proton from the acid stream that passes through the cation exchange membrane to form lactic acid, while sodium chloride is formed in another water stream. This process produces sodium chloride as a side product.
To minimize chemical consumption and to achieve “zero” by-product, bi-polar electrodialysis membrane was proposed. Water splitting reaction occurs at the bipolar membrane, which generates the acidic proton for conversion of lactate to lactic acid and hydroxide ions for sodium cation to form sodium hydroxide. The sodium hydroxide solution is reusable by the fermentation step. Usually the clarified broths are purified and concentrated with conventional electrodialysis membrane first before subjected to bipolar electrodialysis stack.
The largest drawback of electrodialysis membrane is the requirement of high quality feed and the high operating cost associated with the high electric current necessary for fast organic acids transport, and the relatively high cost of the membrane, in particularly, the bipolar membrane. Besides, the selectivity may not always favor the desired outcome.
Another practical recovery technique is reactive liquid-liquid extraction, where the organic acids are being extracted into an organic phase with a suitable carrier. The organic acids are then back extract into aqueous phase. The carrier could be either cationic or neutral.
With neutral carrier, such as tertiary amine, the carrier will extract the organic acids directly, which means, protonation prior to extraction has to be carried out. The stripping aqueous phase can either be water alone or with chemical. The advantage of water stripping is clear. When the organic stripped is in its acid form with minimum impurities, the distribution ratio could be low. This will restrict the feasibility of direct water stripping. Other stripping agents such as sodium hydroxide, sodium chloride, hydrochloric acid etc., can also be used. These stripping agents have high stripping efficiency, but this would mean that there will be high “contaminants” (the stripping agent itself) present in the product and therefore, further purifications steps are necessary. An alternative method is to use a water-soluble tertiary amine as a back extractant. For example, trimethylamine (TMA) can completely back extract the organic acids from the organic phase. The organic acids are then recovered by decomposing the TMA-RCOOH complex at elevated temperature. The TMA is evaporated and collected for reuse, leaving the organic acids behind.
U.S. Pat. No. 6,472,559 B2, discloses the use of phase transfer extraction of lactic acid from aqueous phase to water insoluble amine rich organic phase under highly pressurized carbon dioxide environment. The lactic acid is back extract to aqueous phase after removal of carbon dioxide environment. The drawback of this technique is the use of large quantity of organic solvent.
With cationic carrier, usually in the form of quaternary amine, the carboxylate is exchanged with the counter ions of the amine and thus is extracted into the organic phase. The carboxylate is then stripped with salt or acid, which resulted in organic acid salt and organic acid, respectively, in the end stripping solution. Whichever way, large quantity of the stripping agents (“the contaminants”) will be present in the stripping solution. Further purification steps need to be carried out to remove the contaminants.
Separation by liquid membranes has increasingly caught the attention of the researchers since the 1980s. There are few variants of liquid membranes, i.e., liquid emulsion membrane, hollow fiber supported liquid membrane, and flat sheet supported liquid membrane. Liquid membranes separate the organic acid through liquid-liquid partitioning of the source stream with an organic phase that contains an active carrier. The organic acid is being extracted into the organic phase and it is then being back extracted into aqueous phase through partitioning of the organic phase with the stripping solution. The “separation” mechanism of supported liquid membrane (SLM) is different from the normal membrane. The normal membrane separates components by size, whilst SLM extracts the interest component via chemical mean based on facilitated transport mechanism. The chemistry of SLM is basically liquid-liquid extraction. A significant advantage of SLM over liquid-liquid extraction is that it requires very minimum organic solvent, which result in friendlier operation.
However, the adoption of SLM in real industrial application has been limited by the stability (useful life) of the SLM that generally last only several hours. This is due to the lost of solvent and/or carrier to the aqueous phase. The Water that is being transported across the membrane layer plays an important role in destabilizing the membrane.
It is the object of at least one embodiment of the present invention to provide a complete downstream processing process for recovery and purification of organic acids, in particular, lactic acid from fermentation broth containing lactic acid using supported liquid membrane and other purification technologies.